The present invention relates to a method for modifying a polyvinyl chloride (referred to hereinafter as "PVC") membrane to improve its properties.
Membrane separation processes have become an important unit operation in the chemical industry due to their many advantages. Commercially available separation membranes include cellulose acetate, polysulfone, polyvinylidene fluoride, polyamide, polyimide, polyvinyl alcohol, and polyvinyl chloride. Since PVC resin has excellent chemical resistance and is inexpensive, it is extensively utilized as a high-performance separation membrane. PVC, however, is a hydrophobic material, and the water flux of PVC separation membranes is not high. In addition, it is easy for impurities such as colloids to form sediment on the surface of the membrane during separation operations, resulting in a rapid decrease of the water flux of the membrane.
The most direct and efficient method to overcome these drawbacks is to increase the hydrophilic property of the PVC separation membrane. Several modifying methods have been proposed in the prior art literature. For example: (1) The Journal of Membrane Science, 36 (1988) 187-199, discloses a high frequency discharge process to modify the PVC membrane with inorganic O.sub.2, N.sub.2, H.sub.2, or He atmospheres to enhance the PVC membrane properties. (2) As reported in Desalination, 70 (1988) 277-292, a Co.sup.60 gamma-ray is utilized to graft acrylonitrile onto the PVC, and a phase inversion process is further utilized to prepare the modified PVC separation membrane. This process improves the permeation and separation characteristics of the membrane. (3) In the book entitled Synthetic Polymeric Membranes, Walter de Gruyter & Co., Berlin, Germany, pp. 203-212 (1987), it is mentioned that F. Vigo, C. Uliana and R. Pedemonte utilized a homogeneous heat treatment or a peracetic acid chemical treatment to modify PVC. The modified PVC is then dissolved in N,N-dimethylformamide (DMF) to form a membrane casting solution. The casting solution is used to produce a modified PVC separation membrane by a phase inversion process. Since the resultant membrane has hydroxyl or acetate groups by virtue of the modification of the PVC resin, the hydrophilic property of the membrane is enhanced and the performance of the separation membrane improved.
The above-mentioned methods (1) and (2) are, however, unsuitable for mass production and commercialization, as well as being expensive. In method (3), the PVC resin is subject to a cross-linking reaction during the heat treatment process. This will decrease the solubility of the modified PVC resin in the solvent and adversely affect the preparation of the casting solution. In addition, the modified PVC resin product can be used only after being subjected to complicated purification processes. There thus is a need for a simplified method for improving the properties of polyvinyl chloride separation membranes.